LONDON – Queen Elizabeth II has reached a major milestone by becoming the longest-reigning monarch in Britain's history.

She passes Queen Victoria, her great-great-grandmother, who was on the throne for 63 years and 7 months.

Prime Minister David Cameron said Wednesday that millions on Britons would celebrate the queen's "historic moment."

"Over the last 63 years, Her Majesty has been a rock of stability in a world of constant change and her selfless sense of service and duty has earned admiration not only in Britain, but right across the globe," Cameron said.

"It is only right that today we should celebrate her extraordinary record, as well as the grace and dignity with which she serves our country."